This invention relates to a new process for the preparation of oligomeric polyisocyanates by the oligomerization of a portion of the isocyanate groups of organic polyisocyanates, followed by termination of the reaction with certain silylated acids. The invention further relates to the use of the products of the process (optionally with their isocyanate groups blocked by blocking agents) as polyisocyanate component for the production of polyurethane resins.
The term "oligomeric polyisocyanates" is used in the context of this invention to denote polyisocyanates containing uretdione groups and optional isocyanurate and/or urethane groups obtained from the catalytic dimerization of organic polyisocyanates (preferably diisocyanates) optionally in the presence of alcohol co-catalysts. The urethane groups are formed by the reaction of a portion of the isocyanate groups with the alcohol co-catalysts. Whether the reaction results mainly in dimerization products containing uretdione groups or mainly in trimerization products containing isocyanurate groups depends mainly on the degree of conversion and the temperature control, especially when using tertiary phosphines, which are preferred catalysts. See A. Farkas and G. A. Mills, Adv. Catal. 13, pages 393 et seq (1962).
For reproducible large scale industrial production it is, however, essential to terminate the dimerization and/or trimerization reaction (i.e. the "oligomerization reaction") precisely and rapidly at a predetermined point.
According to German Offenlegungsschrift 3,432,081, the course of the reaction is controlled by inactivation of the catalysts by means of sulfonyl isocyanates, particularly tosyl isocyanate. Tosyl isocyanate is superior as catalyst poison to the previously known catalyst poisons discussed in German Offenlegungsschrift 3,432,081, but the adducts formed by reactions between tosyl isocyanate as catalyst poison and the catalyst are obviously still so unstable that back formation of the catalyst may take place. Therefore, it is recommended to use more than equimolar quantities of sulfonyl isocyanate. The addition of equivalent quantities of tosyl isocyanate, based on the quantity of catalyst used, is indeed insufficient for reliable stopping of the reaction, as the comparison experiment described below will show.
It was, therefore, an object of the present invention to provide a new process for the oligomerization (that is, dimerization and/or trimerization) of organic polyisocyanates, preferably diisocyanates, using catalysts known in the art, whereby the reaction can be accurately stopped at the desired degree of conversion. This problem has been solved by the process according to the invention described below.